In, for example, highly loaded train platform cells there are occasionally reasons for combination of platform cells into larger cell clusters/groups. In particular, large groups of users simultaneously moving might create more control signaling per time instance, and thus cause heavier load on involved network nodes. Combining/clustering of adjacent cells will reduce the load associated with communication directly or indirectly between the cells or between the user equipment and neighboring cells.
With Long Term Evolution (LTE) release 8 User Equipment (UE) the cell can be combined to a transmitting the same information from all the transmission points in the cell. This is also called “single frequency network” and is typically used along roads and railroads.
Combining cells reduces the number of cells, the area splitting gain and thereby also reduces capacity. In LTE release 10 a new transmission mode 9 (TM9) is introduced. TM9 supports UE specific demodulation reference symbols (DMRS) and Channel State Information Reference Signal (CSI-RS) enabling spatial multiplexing within the combined cell.
Traditionally, there has been a one-to-one correspondence between antenna/transmission points and cells leaving limited possibilities for cell combinations. Distributed antennas solutions to obtain good coverage with equal signal levels have emerged where many transmission points are used. Multiple transmission points are often combined/clustered to/as single cells or similar. Examples are active Distributed Antenna System (DAS), Radio Dot System (RDS) and small cells Remote Radio Unit (RRU) clusters.
In the published U.S. Pat. No. 8,331,236 B2, a method to balance traffic load between nearby LTE/WiMAX cells grouped into inner and border constellations is disclosed.
Within Coordinated Multi Point (CoMP) there are several methods suggested on how to group UEs and transmission points to be used for joint transmission and reception. For example, in the published international patent application WO 2012/150881 a distributed method how to interact between nodes to combine antennas and transmission points controlled by different nodes is described. And in the published international patent application WO 2012/126514 a method on grouping UEs and transmission points for CoMP transmission based on path loss is described.
Even though there are advanced CoMP features that in theory can avoid cell split between any transmission points the size of a CoMP cell is in practice limited due to signaling and processing capabilities. Also, existing combined cell solution based on LTE transmission modes 9 and 10 do not reach the capacity as of individual cells due to increased overhead.
Combining all cells (with sufficient backhaul) will reduce capacity. This is true not only for LTE Release 8. DMRS increases overhead and reduces capacity and peak bitrate.
External input from e.g. vehicle traffic control system is described for channel allocation in the published US patent application US 2002/0016170 A1, but no solution on combined cell and how cell-id should be broadcasted is described.